This study is designed to examine polysomnographic parameters, especially REM latency, as a vulnerability factor for depression in "at risk" individuals. First-degree relatives of identified patients with Major Depressive Disorder are considered to be at increased risk for developing depression, compared to the population as a whole. Outpatients and inpatients aged 20-40 will be identified and carefully diagnosed through the Affective Disorders Unit. These patients will be studied for two consecutive nights in the sleep laboratory of Southwestern Medical School and assigned to one of two groups as defined by their mean REM latencies: Group 1 - REM latency less than or equal to 65.0 minutes (n=15); Group 2 - REM latency greater than 65.0 minutes (n=15); Group 3, to be comprised of normal subjects (n=15), aged 20-40, with nonreduced REM latencies and no personal or family history of psychiatric disorder, will be developed for comparison with the depressed groups. Four first-degree relatives, two siblings and two parents, for each depressed and normal control subject will be assessed in the sleep laboratory and by structured psychiatric interview. The major dependent variables to be measured in these subjects are (1) polysomnographic parameters, especially REM latency and (2) personal history of affective disorder. Thus, each of the three cells (reduced REM latency cell, nonreduced REM latency cell, normal control cell) will contain 60 first-degree relatives. We predict that there will be a) an increased incidence of first-degree relatives with reduced REM latencies in families of reduced REM latency depressed probands, and b) relatives with reduced REM latencies will have a higher incidence of positive history for affective disorders. As a consequence of completing this research, a sample of asymptomatic subjects at risk for depression, but with no personal history of affective disorders, will de developed for longitudinal prospective study.